Cam phasers create a variable rotational offset between an exhaust camshaft, an intake camshaft, and a crankshaft of an internal combustion engine (ICE). The degree of rotational offset generated by the cam phaser enables the ICE to be tuned for specific performance requirements by varying valve overlap, i.e., overlap between the exhaust and intake valves of the ICE. In applications where idle quality is important, a relatively small degree of valve overlap may be desired. In applications where nitrogen oxides (NOx) must be reduced, a relatively large amount of overlap may be desired. The cam phaser may provide charge dilution in the form of recirculated exhaust gases. Charge dilution is a method of adding a non-reacting substance to the air/fuel mixture in a cylinder of an ICE to decrease the heat capacity of the air/fuel mixture and thus reduce the amount of NOx components.
A cam phaser typically includes a cylindrical stator and a vaned rotor. The stator is mounted onto a crankshaft driven gear or pulley and typically has a plurality of radically-disposed inward-extending spaced-apart lobes and an axial bore. The rotor is mounted to the end of the camshaft through the stator axial bore and has vanes disposed between the stator lobes to form actuation chambers such that limited relative motion is possible between the stator and the rotor.
The cam phaser is provided with suitable porting so that hydraulic fluid, for example, engine oil under engine oil pump pressure, can be brought to bear controllably on opposites sides of the vanes in advancing and retarding chambers. Control circuitry and valving permit the addition and subtraction of oil to the advance and retard chambers. Changes in rotational phase between the stator and rotor cause changes in timing between the pistons and the valves.
Under conditions of low engine oil pump pressure, such as during startup, it is desirable to mechanically lock the rotor and stator together in a default mode to prevent unwanted angular movement of the rotor relative to the stator. This is typically accomplished by a hydraulically activated lock-pin disposed in the rotor and positioned parallel to the rotational axis of the stator. When the oil pump pressure reaches a predetermined level, the hydraulic force of the oil causes the locking pin to retract from the pin bore and into the rotor. As a result, the rotor is decoupled from the stator and cam shaft phasing can occur.
In an effort to reduce rattling noise during startup, clearance between the lock-pin and pin bore is very tight. This tight clearance increases the potential for the cam phaser to move before the lock-pin is fully retracted. The result is a side load on the lock-pin that is sufficient to prevent it from further retracting and thus the cam phaser remains stuck until the side load on the lock-pin is removed.
In one method, a control module cycles the cam phaser solenoid for a fixed number of cycles to remove debris from the valve when a performance diagnostic determines that the solenoid valve is stuck. This solution is acceptable for removing debris, but it is not well suited to address the condition of a stuck lock-pin.